There are many applications where filtering is required with a minimum of delay through the filter. One such application is in real-time control systems used for active noise cancellation.
Modern control systems often use sampled-data filters which may be analog devices such as charge coupled devices but are more often digital filters. There are inherent delays associated with a sampled-data filter. For example, in a digital filter these are due to the anti-aliasing filters (which may be digital or analog), the anti-imaging filters (which may be digital or analog), the sample and hold of the Digital to Analog Converter (DAC) and the computation time of the digital processor.
The first three of these are related to the sampling period of the digital filter. They can be reduced by increasing the sampling rate of the digital filter. However, this requires an increase in the computational power of the processor and results in increased cost and electrical power consumption.
For some applications a fixed filter is sufficient, and there have been attempts in the past to modify digital filters by combining them with analog components.
It is known that the delay associated with the filter can be avoided if the digital filter is combined with an analog circuit in such a way that there are a least two paths through the filter, one of which avoids the digital filter.
For example, in UK patent GB2142091, `Attenuation of sound waves`, Swinbanks describes a filter, shown in FIG. 1, which comprises a fixed analog gain with digital FIR filter in the feedback path and in UK patent application GB2222733, `Improved digital filter with an analog path`, Harper and Ross describe a fixed analog filter with a digital FIR filter in a parallel path. This is shown in FIG. 2.
For most practical applications however, it is necessary to adjust the overall response of the filter in order to maintain the best control performance in a changing environment. There are no previously known methods for adapting combined filters of the type described by Swinbanks or by Harper and Ross.
There is some discussion by Swinbanks of how to determine the digital filter coefficients when a fixed-gain analog amplifier is used, but this is only possible if the characteristics of the system to be controlled are fixed and can be determined in advance. There is no discussion of how to choose the gain of the analog amplifier.
While there is some discussion by Harper and Ross of how to determine the desired characteristic of the analog filter when the digital filter is fixed, there is no discussion of how to determine the characteristics jointly.
The desired response can be more closely matched by designing the two filters together. Additionally, it is often desirable to use a fully adaptive control system for practical control applications. This avoids the need to redesign the system for each new application and allows the control system to be used when the system to be controlled varies With time.
The digital filters described by Swinbanks and by Harper et al are for use with feedforward active control systems. For this type of control system an `upstream` sensor is used to obtain an advance measurement of the noise to be canceled. In this application, the characteristic of the digital filter depends upon the acoustic response of the physical system and on the characteristics of the actuator. These may be slowly varying over time. However, feedback control systems do not use an `upstream` sensor, instead they only use sensors in the region where noise control is required. In this case the information received from the sensor is generally too late, and the control system must be able to predict the noise. Any delay through the filter will make this prediction more difficult. The prediction is strongly dependent upon the characteristics of the noise, and so the filter characteristics must be varied as the noise varies. Hence, active feedback control requires a fully adaptive control system with a low throughput delay.
Accordingly, it is an object of this invention to provide a filter with a low throughput delay without using a processor.
Another object of this invention is to provide a filter means having multiple paths therethrough, one of them being a higher speed path than at least one of the others.
A further object of this invention is to provide a parallel path filtering means wherein the characteristics of the filters are selected to minimize the difference between overall filter response and the desired response.
A still further object of this invention is to provide a method of avoiding filter delays in an active noise cancellation circuit.